Light emitting diodes (LEDs) based on InAlGaN multiple quantum wells (MQWs) generally suffer from certain limitations in terms of energy efficiency to produce light. One limitation is to efficiently extract light generated in the MQWs from inside the LEDs to air. Due to the refractive index difference between the InAlGaN material and air, a majority of the photons generated inside the MQWs is usually trapped inside by total internal reflection (TIR) and cannot escape the LEDs. Such photons eventually are lost inside the LEDs due to intrinsic material losses and other absorption mechanisms.
There has been various ways being developed to improve light extraction efficiency (LEE) from a LED device. For LEDs grown on absorbing layers/substrates, window layers have been used to help extract light from side cones of a LED chip, and distributed Bragg reflectors (DBRs) have been applied to reflect light away from the absorbing layers.
For LEDs grown on transparent substrates, roughening/texturing of the p-type layers has been used to improve light extraction for epi-side up structures. For flip-chip LEDs, post epi-growth processing is required, including substrate removal and roughening/texturing of the n-type layers (thin film flip-chip), or thinning down the substrate (for example, sapphire substrate) to less than 100 um. For vertical structure LEDs, after substrate removal, photonic crystals have been employed to pattern the n-type layers to achieve high LEE.
Despite significant advances achieved in improving the LEE, most of the existing approaches require post epi-growth processing which can be quite complex and potentially affects the yield, reliability and cost of the LEDs. A simple method that does not require extensive post-growth processing of the LED wafers, yet helps to improve LEE would be highly desirable, especially in the manufacturing environment.
Patterned sapphire substrate (PSS) has been well established as a substrate of choice for LED epitaxy. The epi growth proceeds by deposition of InAlGaN material onto a substrate with pre-defined features. Such features are usually defined onto the surface of the substrate by wet or dry etching, characterized by their height, width, length, diameter and/or pitches, and are usually periodic in nature. Thin film epitaxy, especially LEDs, onto such features will produce epi materials with reduced dislocation density and thus improvement in power output. Also, such features acts as scattering centers in the material, which improves the LEE of the LED by increasing the opportunity for light to escape from the InAlGaN into the substrate from multiple bounces.